372 SPECIAL HISTOLOGY. 



new lainellse, like the primary one, are at first perforated 

 by reticular openings, and the various sized, roundish or 

 elongated interstices communicate with those of the pre- 

 viously and subsequently formed layers, so that the secondary 

 osseous nuclei, like the periosteal layers, are, from the 

 first, penetrated by a network of canals, which, as in those 

 layers, in part at least, soon present the appearance of Haver- 

 sian canals. At first, filled only with a soft blastema, the 

 remains of the plastic material of the various lamellse, these 

 spaces, in consequence of the advance of ossification in their 

 interior, — which sometimes takes the form of bridges stretch- 

 ing across them, sometimes of a deposit on their walls, — 

 become more and more contracted. Ultimately, some are 

 entirely closed, whilst others are converted into true vascular 

 canals, the vessels being developed from their contents, which 

 are composed during this time of medulla-cells, and com- 

 municating with those of the periosteum. When the bone has 

 arrived at this stage, its subsequent changes are readily 

 followed. It continues to increase in breadth and thickness 

 by the constant addition of new blastema on its edges and 

 surfaces, until it has attained its typical form and size, and at 

 the same time, by the solution of its compact substance, 

 additional spongy tissue (or even large cavities), is formed in 

 its interior, so that eventually, like bone developed from 

 cartilage and periosteal layers, it presents, externally, compact 

 substance with Haversian canals; and internally, medullary 

 spaces (cancelli), although with distinct secondary deposits. 



[The secondary cranial bones ossify, in part, earlier than the 

 primary, and mostly with only a single nucleus. The soft 

 blastema out of which they are formed, and which, so long as 

 the bones continue to grow, is to be found on their surfaces 

 and edges, does not, like cartilage, grow independently 

 with them, but is developed by degrees, from a plasma 

 successively secreted from the vessels of the periosteum, the 

 two lamellse of which are conjoined at the margin of the 

 ossifying plate. The cells of this plasma, the metamorphosis 

 of which, as iu the periosteal layers, cannot be followed in 

 every particular, are elongated, measuring in man, for the 

 most part, 006 — - 01'", and presenting granular contents 



